Technical Field
The invention relates to a power conversion technique. Particularly, the invention relates to a ramp signal generating method and a generator thereof applied to a power converter, and a pulse width modulation signal generator.
Related Art
FIG. 1 is a schematic diagram of a conventional power converter. FIG. 2 is a waveform diagram of the conventional power converter. Referring to FIG. 1 and FIG. 2, the conventional power converter 100 generally adopts a constant on time architecture. A ramp generator 140 generates a ramp signal Xramp having a constant triangular wave. A comparator 110 compares an error signal Xerr with the ramp signal Xramp to generate a comparison signal Xcm. A time control circuit 120 generates a pulse width modulation (PWM) signal Xpwm according to the comparison signal Xcm, an input voltage Vin and an output voltage Vout. A width of an on time Ton of each period of the PWM signal Xpwm is a constant value, and the width of the on time Ton relates to the input voltage Vin and the output voltage Vout.
In the power converter 100, the comparison signal Xcm is generated according to the error signal Xerr and the ramp signal Xramp. The time control circuit 120 determines the on time Ton for outputting the PWM signal Xpwm according to the comparison signal Xcm. An amplitude of the error signal Xerr relates to a feedback signal Vfb and a reference voltage Vref. At a moment for deciding the on time Ton for outputting the PWM signal Xpwm, the time control circuit 120 starts to calculate and generate the on time Ton, and the on time Ton of each period of the PWM signal Xpwm is constant.
Although the conventional operation architecture of pulse width modulation may achieve an effect of fixed frequency, when an equivalent series resistance ESR of a capacitor CL and an equivalent series resistance DCR of an inductor L on an output terminal of the power converter 100 are all very small, the energy compensated by the capacitor CL and the inductor L in response to a load transient variation is delayed, so that the feedback signal Vfb and the error signal Xerr are also delayed. The original error signal Xerr generated by the compensation circuit 130 cannot be used to converge the output voltage Vout. Moreover, since the ramp signal Xramp has a waveform with a fixed discharge slope, the discharge slope cannot be changed along with the load transient variation. The above reasons lead to unstable oscillation of the power converter 100.
FIG. 3 is a circuit diagram of the conventional ramp generator. Referring to FIG. 1 and FIG. 3, in the ramp generator 140, a current source IRamp, N-type metal oxide semiconductor (MOS) transistors MN1 and MN2 construct a current mirror. A first clamping voltage Vclamp1 is greater than a second clamping voltage Vclamp2. When the error signal Xerr is greater than the ramp signal Xramp, a reset signal RST turns on a switch S1, and a voltage of a capacitor Cramp is charged to the first clamping voltage Vclamp1. Then, the current mirror discharges to the capacitor Cramp. A current value of the current source IRamp (a discharge current) is fixed. When the ramp signal Xramp is discharged to the second clamping voltage Vclamp2, the ramp signal Xramp is clamped to the second clamping voltage Vclamp2. The ramp generator 140 provides the ramp signal Xramp having a constant waveform through the aforementioned charging and discharging operations to serve as an adjustment reference for comparing with the error signal Xerr.
FIG. 4 is another waveform diagram of the conventional power converter. Referring to FIG. 1 and FIG. 4, related waveforms of FIG. 4 are obtained when the power converter 100 adopts the constant on time architecture and is operated in a discontinuous conduction mode (DCM). When an output load current Iload belongs to a very light load, an operation frequency of an inductor current IL is decreased (i.e. a time of the operation period is prolonged), such that the ramp signal Xramp having the fixed discharge slope is discharged to a level of the lowest clamp voltage. When the output voltage Vout has insufficient energy and requires energy, the waveform of the error signal Xerr climbs up, and an angle θ formed between the climbed error signal Xerr and the approximately horizontal ramp signal Xramp is decreased. The angle θ relates to anti-noise capability. When the angle θ is decreased, the anti-noise capability of the power converter 100 is decreased, and severe jitter is probably generated.